2798
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Table 4. Typical Correlations between UCS and shaft friction
TP1 –
1.64 MPa
TP2 –
1.48 MPa
Design Method
Equation
UCS
(MPa)
Percentile
(Pearson5
/ Normal)
UCS
(MPa)
Percentile
(Log-Normal
/ Normal)
Hovarth and
Kenny (1979)
62.6
80% / 85%
51.2
70% / 50%
Meigh and
Wolski (1979)
28.3
70% / 45%
24.0
35% / 30%
Williams,
Johnson and
Donald (1980)
20.5
(
=
0.1)
(
= 0.8)
60% / 35%
22.8
(
= 0.1)
(
= 0.65)
30% / 25%
Rowe and
Armitage (1987)
13.1
60% / 30%
10.8
< 5% / 20%
66.6
85% / 85%
54.5
70% / 50%
(Lower Bound Equation)
13.1
50% / 30%
10.7
< 5% / 20%
Carter and
Kulhawy
(1988)
(Upper Bound Equation)
52.9
(C = 1)
85% / 75%
43.3
(C = 1)
60% / 40%
Kulhawy and
Phoon
(1993)
13.2
(C = 2)
50% / 30%
10.8
(C = 2)
< 5% / 20%
Prakoso (2002)
26.5
70% / 45%
21.6
25% / 25%
The results indicate that various researchers appear to have
assumed a Normal distribution in developing shear capacity
formulae, with a lower quartile to mean / median value
generally adopted (20
th
to 50
th
percentiles). Higher (≥50
th
)
percentiles were required to replicate the observed ultimate
capacity values for lower bound (conservative) pile capacity
formulas. As the adopted design UCS value is commonly above
the point of equivalency between the Normal and non-normal
distribution (
q
uc
25
th
percentile, refer Table 2), the
comparable back-calculated design strength percentiles are
generally higher for the non-normal distributions.
However, the more accurate distribution function has been
shown to be non-normal. Using the best fitting distribution, the
derived UCS values required to replicate the shaft capacity
observed in TP1 were consistently at, or above, the median
value. This suggests that all considered design methodologies
would, if the non-normal 50
th
percentile value was adopted,
provide overly conservative shear capacity values for this site.
To avoid the inconsistencies associated with use of incorrect
distribution functions a characteristic
q
uc
value about the 20
th
to
30
th
percentile range was previously recommended. Using this
percentile range of the Normal and non-normal TP1 rock
strength (UCS) datasets, or the larger Pier 6 datasets (also
assumed representative of TP1), the formula provided by Rowe
and Armitage (1987), and upper bound equations from Carter
and Kulhawy (1988) and Kulhawy and Phoon (1993) calculate
pile shaft capacities closest to those observed. In the higher
strength rock profile of TP2, the capacity equations provided by
Meigh and Wolski (1979) and Prakoso (2002) displayed the
closest match to the observed shaft capacity when the 20
th
, 25
th
or 30
th
percentiles of the UCS datasets were adopted.
4 CONCLUSIONS
Statistical analysis of the available GUP rock strength data
shows that if a Normal distribution is assumed for characteristic
value determination, then errors may result. To minimise
inconsistencies associated with the use of ill-fitting distribution
functions to describe strength data, then the selection of values
near the lower quartile of the UCS dataset is recommended.
Two large-scale instrumented test piles were loaded beyond
shaft capacity at the GUP site. Based on this test data, the
required input UCS value has been back calculated for a number
of pile design methods, and the indicative strength percentile
reliability of the UCS value has been determined. Five of the
examined methods have produced results that match the
observed shaft capacities via the adoption of a design UCS
value close to the UCS lower quartile “characteristic” value.
5 ACKNOWLEDGEMENTS
Some reliability concepts were formulated while the principal
author was working on this project at Connell Wagner Pty Ltd
together with Vasanatha Wijeyakulasuriya of Queensland Main
Roads. Fugro-Loadtest undertook the field trials discussed.
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